Seismic Design of Buried Structures Doug Jenkins Interactive Design Services.
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Seismic Design of Seismic Design of Buried Structures Buried Structures Doug Jenkins Doug Jenkins Interactive Design Services Interactive Design Services
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Transcript of Seismic Design of Buried Structures Doug Jenkins Interactive Design Services.
Seismic Design of Buried Seismic Design of Buried StructuresStructures
Doug JenkinsDoug JenkinsInteractive Design ServicesInteractive Design Services
Why Bother?Why Bother?
"Results show that the bending moments in the arch from horizontal earthquake loading can be significant in relation to the gravity load actions. These moments are also very sensitive to the backfill and surrounding soil stiffness properties and rather less sensitive to the foundation soils beneath the arch.”
John Wood
Collapse of buried structure in JapanCollapse of buried structure in Japan
Spec
Possible earthquake related failure south of Sydney Consequences of failure
Seismic design provisions in AS5100Seismic design provisions in AS5100
Specific rules are given for the categorisation of bridges. A formula is given for the fundamental period of bridge
structures (for use in category BEDC-1 designs only). Specific structural response factors are given for bridges of
different types. Structural detailing requirements relevant to bridges are
given.
Problems with application to buried Problems with application to buried structuresstructures
The requirements for more detailed analysis methods are related to bridge span, and may not be relevant to buried structures.
Vertical earthquake effects may be important for buried structures, but only horizontal effects are required to be considered for almost all buried structures.
The formula for the fundamental period is not applicable to buried structures.
For static analysis the earthquake design force is not applicable to buried structures.
The appropriate response modification factor is not clear.
Buried arch design studyBuried arch design study
Geraldton Southern Transport Corridor
14.5 m span x8 m high 3m and 15 m cover Acceleration 0.10g Site factor 1.5 Bridge Type II
ClassificationClassification
Type III - Bridges and associated structures that are essential to post-earthquake recovery, as determined by the relevant authority.
Type II - Bridges that are designed to carry large volumes of traffic or bridges over other roadways, railways or buildings.
Type I - Bridges not of Type II or Type III.
ClassificationClassification
Bridge classification Product of acceleration coefficient and site
factor Type III Type II Type I
aS ≥ 0.2
BEDC-4 BEDC-3 BEDC-2
0.1 ≤ aS < 0.2
BEDC-3 BEDC-2 BEDC-1
aS < 0.1
BEDC-2 BEDC-1 BEDC-1
ClassificationClassification BEDC-1
– < 20 m span, no earthquake analysis
– > 20 m span, static analysis, horizontal forces
BEDC-2– Static or dynamic analysis
– > 35 m span, horizontal and vertical
BEDC-3– Static analysis only if a single dominant mode
– Consider horizontal and vertical loads
BEDC-4– Dynamic analysis; horizontal and vertical
Buried arch design studyBuried arch design study
For each fill height (3m and 15 m):– Natural frequency analysis– Deflection under unit horizontal acceleration– Pseudo-static analysis– Response Spectrum analysis– Push-over analysis
Buried arch design studyBuried arch design study For each of the analyses the following
material stiffness properties were used:
– Typical soil and uncracked concrete.
– Soil stiffness reduced by half and uncracked concrete.
– Soil stiffness reduced by half, and concrete moment-curvature relationship (static analyses), or cracked stiffness (response spectrum analyses)
A total of 26 separate analyses were carried out.
Typical Finite Element MeshTypical Finite Element Mesh
Mesh DetailMesh Detail
3 metres cover Period, T 0.055 0.5 Stat T /Dyn T C
Elastic 0.605 119 0.600 99.1% 0.261 Reduced soil 0.850 231 0.836 98.4% 0.209 Reduced both 0.853 233 0.840 98.4% 0.208
15 metres cover Elastic 0.999 341 1.016 101.7% 0.188 Reduced soil 1.405 679 1.433 102.0% 0.149 Reduced both 1.408 680 1.434 101.8% 0.149
Fundamental period, Static Fundamental period, Static Design Force CoefficientDesign Force Coefficient
Static analysis, 3m CoverStatic analysis, 3m Cover
-200
-150
-100
-50
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Distance around arch, m
Ben
din
g M
om
ent,
kN
m
Standard stiffness Reduced soil stiffness Reduced soil and beam stiff
Dynamic Analysis, 3m CoverDynamic Analysis, 3m Cover
-250
-200
-150
-100
-50
0
50
100
150
200
250
0.0 5.0 10.0 15.0 20.0 25.0
Distance around arch, m
Ben
din
g M
om
ent,
kN
m
Dynamic Lin Dyn, reduced soil stiffness Dyn, reduced soil&beam stiff
Maximum Moments, 3m CoverMaximum Moments, 3m Cover
-250
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0
50
100
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200
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Standard Stiffness Reduced Soil Stiffness Reduced Soil and Beam
Ben
din
g M
om
ent,
kN
m
Pseudo-Static, +ve Resp. Spect. +ve Pseudo-Static, -ve Resp. Spect. -ve
Static analysis, 15m CoverStatic analysis, 15m Cover
-250
-200
-150
-100
-50
0
50
100
150
200
0.0 5.0 10.0 15.0 20.0 25.0
Distance around arch, m
Ben
din
g M
om
ent,
kN
m
Standard Reduced soil stiffness Reduced soil and beam stiff
Dynamic Analysis, 15m CoverDynamic Analysis, 15m Cover
-250
-200
-150
-100
-50
0
50
100
150
200
250
0.0 5.0 10.0 15.0 20.0 25.0
Distance around arch, m
Be
nd
ing
Mo
me
nt,
kN
m
Dynamic Lin Dyn, reduced soil stiffness Dyn, reduced soil&beam stiff
Maximum Moments, 15m CoverMaximum Moments, 15m Cover
-250
-200
-150
-100
-50
0
50
100
150
200
250
Standard Stiffness Reduced Soil Stiffness Reduced Soil and Beam
Ben
din
g M
om
ent,
kN
m
Pseudo-Static, +ve Resp. Spect. +ve Pseudo-Static, -ve Resp. Spect. -ve
Axial Load - Seismic Increment Axial Load - Seismic Increment or reduction, 15m Coveror reduction, 15m Cover
15m Cover Axial Loads, Seismic Increment
0
200
400
600
800
1000
1200
0.0 5.0 10.0 15.0 20.0 25.0
Distance around arch, m
Axi
al F
orc
e, k
N
Pseudo Static Response Spectrum
AnimationsAnimations
First mode shape - 3m and 15 m cover, horizontal and vertical– Arch + Fill– Fill only
Moment-CurvatureMoment-Curvature
Push-over analysis, 15m CoverPush-over analysis, 15m Cover
-800
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-200
0
200
400
600
800
0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0
Crown X Displacement, mm
Ben
din
g M
om
ent,
kN
m
Min Moment Max Moment
AnimationsAnimations
Push-over analysis– 15 m fill– 3 m fill
ConclusionsConclusions
Buried arch structures with low to moderate axial loads have a large reserve ductility.
Where the failure mode is concrete compression failure under moderate earthquake loading is a possibility.
Bending moments found in the dynamic analyses were up to 50% higher than the static analysis results.
RecommendationsRecommendations Structure classification be related to fill height, rather than
span. Response modification factor to be related to the capacity
reduction factor. BEDC-1 and BEDC-2: design for vertical and horizontal
earthquake loads, using static or dynamic analysis. BEDC-3 and BEDC-4: design for vertical and horizontal
earthquake loads, using dynamic analysis. Determine the fundamental period from computer analysis
or formula for BEDC-1 and 2 structures.
– T = 0.055 0.5 seconds = deflection in millimetres at ground surface above the arch
under 1g horizontal load.
Further InformationFurther Information
www.interactiveds.com.au
